The invention relates to a microwave sensor for measuring a dielectric property, having a microwave resonator, wherein a product introduced into the resonator interacts with a resonant microwave field generated in the resonator in order to determine suitable measured quantities related to, for example, density and/or moisture content of a product. The invention further relates to use of the sensors in a spinning preparation machine.
With such sensors, the field energy is concentrated in a specific region of the resonator. The mass loading of the resonator in the region of high field energy is limited, since too great a mass loading leads to falsification of the measuring signal owing to losses. The use of several resonators, each one only for a portion of the product, is expensive and can lead to systematic errors owing to differences in the resonators and in the corresponding control and evaluating electronics. In the case of extended, especially web-form, products, a further problem comprises the sensing of the product over a relatively large dimension, for example, across the entire width of a product web. The enlargement of the region of high field energy, for example, by enlarging the wavelength, would lead to constructions of impractical dimensions. The use of several relatively small resonators across the expanse of the product is likewise disadvantageous for the above-mentioned reasons.
EP 0 889 321 A discloses a microwave sensor for measurement on an elongate sample, having a flat, dielectric-filled resonator that has a through-bore running through it for the sample. In the measuring region arranged centrally in the resonator there is an approximately homogeneous microwave field of relatively low intensity.
DE 102 04 328 A discloses a microwave sensor for determining the mass of a fibre sliver in a spinning preparation machine, the sensor comprising a resonator having two sample volumes for two fibre slivers.
WO 00/55606 A discloses a microwave sensor having a resonant strip transmission line for measuring the mass of a fibrous material. A half-wave of the electric field develops along the strip transmission line with intensity maxima at both ends, where there is a respective passage for a flow of product. The highly inhomogeneous fields at the open line ends are disadvantageous in terms of measurement techniques. Furthermore, it is necessary to shield the strip transmission line using a metallic enclosure in order to prevent falsification of the measurement through radiation loss. The enclosure has to be carefully adjusted in order to avoid resonance close to the measuring frequency. The construction of the sensor is therefore comparatively expensive.
It is an aim of the invention to provide an inexpensive microwave sensor having an improved measuring accuracy, and preferably to permit substantially exact measurement of a relatively large product quantity and/or an extensive product.